Selector for diversity receiving systems



Patented May 12, 1942 .i

SELECTOR FOR DIVERSITY RECEIVING SYSTEMS John B. Moore, Riverhead, N.Y., assignor to Radio Corporation of America, a corporation of DelawareApplication June 29, 1940, Serial No. 343,091

9 Claims. (Cl. Z50-20) This invention relates to radio receiving systemsand more particularly to an arrangement for diversity receptionemploying a plurality of antennas and receivers which control a commonutilization device or circuit.

As is well known, radio signals are subject to fading, which varies bothin frequency and degree in an unpredictable manner. Inasmuch as thefading at any instant may differ very widely at geographically spacedpoints or in different planes of polarization, it has been found to be apractical expedient to provide a number of antennas whose energies whencollected are fed selectively to a single responsive means.

It is an object of the present invention to` improve upon diversityreceiving systems heretofore known.

It is a more specific object of my invention to provide a diversityreceiving system which shall be effective in reducing to a minimum the'fading and noise present in the output from a receiver.

It is another object of my invention to provide a switching system in adiversity receiver such that the switching action from one to another ofthe operating antennas will be complete, and substantially instantaneousin its effect.

It is still another object of my invention to provide a switching systemfor diversity receivers which will operate ln response to a very smalldifference in input voltages impressed on two or more of the componentradio receivers supplying the common responsive means, or utilizationcircuit.

The use of an automatic gain control system common to two or morereceivers results in the instantaneous output signals from the separatereceivers being proportional to the instantaneous inputs delivered tothese receivers by the separate signal sources, or antennas. Thispermits the strongest one (having the best signal-tonoise ratio) topredominate at any instant. To further accentuate this inherentswitching action, use is made of relatively low impedance diodes asfinal detectors to supply the common load circuit. Experience and theoryhave both shown that such low'impedance diodes (instead of relativelyhigh! resistance detectors of thel biased triode type) give a ldecidedswitchingV action in themselves when working into a com# mon resistanceload of given value. This is due to the voltage regulation inherentk insuch a circuit. A zero impedance diode source would give 100% completeswitching for an infinitesimal difference in input voltages.

To prevent the overall distortion which certain types of fadingconditions produce in a combined output from two4 or more receivers of adiversity system, complete switching is provided by means of relaysoperated in accordance with the differences between rectified outputsfrom the two or more individual receivers.

Certain systems heretofore in use have employed a common automatic gaincontrol supply from and to the two or more receivers; and have alsotaken advantage of the further im'- provement in switching actionresulting from the use of relatively low impedance final detectorssupplying the common load circuit.

In order to obtain the desired switching action certain other systems inuse have relied entirely on variations in signal voltages at theantennas and on the use of relays to lchoose between the individualoutputs of the several receivers.

Systems of the first type do not make avai1- able the possibility of100% switching on small differences in signal strengths at the severalantennas. The result, under certain specific fading conditions, is anout-of-phase .addition of the modulation frequency components of theoutputs of the several detectors. This results in distortion.

kSystems of the second type, which employ automatic gain control on eachindividual receiver, but which rely on relay switching(electro-mechanical or electron tube) instead of employing an automaticgain control system common to the several receivers, have thedisadvantage of high noise level in the output of any receiver having amomentarily, poor signal-tonoise ratio due to the signal supplied by itsseparate antenna having faded to a low value. Choice must then bemadebetween two or more receiver outputs of approximately the samevoltage-due to individual Vautomatic gain control systemsbut of greatlydifferent signal-to-noise ratios.

The system of this present invention retains the proven advantages of anautomatic gain control supply common to the several receivers: therebypermitting the signal from the receiver having the best signal-to-noiseratio to predominate. The arrangement and functioning of the circuitherein disclosed then selects this predominantly best of the outputsignals from the two or more receivers in use.

A further advantage of the system herein pronosed is that the selectionis effective, in its functioning, whether or not modulation is present.It is, therefore, ready at every instant to vided in combination forcarrying out the invention.

Referring to the drawing, I show two antennas I and 2 each feeding to anappropriate radio frequency amplifier.4 Amplifier 3 is connected toantenna I, while amplifier 4 is connected to antenna 2. I have shownalso a pair of heterodyne converters 5 and 6 each appropriatelyconnected to its own R. F. amplifier. Each converter feeds to itsassociated intermediate frequency amplifier I or 8. The output from eachI. F. amplifier is then fed to its associated diode detector 9 or I9.The rectified intermediate frequency energy. from diode detector 9 iseffective for three purposes: (1) to control an audio frequencyamplifier I3; (2) to provide a suitable automatic switching voltageobtained from the voltage drop across resistor I5; and (3) to provideautomatic gain control from the voltage drop across the common loaresistor I'I.

In like manner the rectified output energy from the diode detector Iproduces a voltage drop across resistors I2, I6, and II.

Resistors I and I6 are both connected to one terminal of a common loadresistor I'I, the other terminal of which is grounded. A commonautomatic gain control voltage is, therefore, provided by the timeconstant circuit connected across resistor I8. Voltage supplied throughresistance I 8 to capacitor 20 is fed through connection I9 to theconventional grid biasing circuits for the R. F. amplifiers 3 and 4, aswell as the I. F. amplifiers I and 8.

In addition to the primary function of applying automatic gain controlvoltages through the circuits IS to the several amplifiers, a switchingfunction is provided by the voltages appearing as drops across resistorsI5 and I6.

The switching system is differential in its action. T he apparatusincludes preferably a twin triode tube 2I having a common cathode 22connected to the junction point between resistors I5 and IB. This tubehas two grids 23 and 24 and two anodes 25 and 2S. The grid 23 isconnected through resistor 21 to the terminal of resistor I5 remote fromits connection with the common cathode 22. correspondingly, grid 24 isconnected through resistor 28 with that terminal of resistor I6 which isremote from the common cathode 22. Time constant condensers 29 areprovided from grids 23 and 24 to the common cathode 22, to suitablydetermine the speed of the switching action and to make said switchingaction non-responsive to modulation frequencies which are to beutilized.

Tube 2l has two output circuits, one extending from the cathode 22through an anode supply source 39 and one portion of a potentiometer 3lthrough resistor 32 to the anode 25. The other output circuit includessaid source 30, the other portion of the potentiometer 3l and resistor33 which is connected to the anode 26.

The twin triode tube 2| operates as an amplilier for feedingdifferential potentials to the grids 34 and 35 respectively of anothertwin triode tube 36. The cathode 5l of this tube constitutes the commonconnection between the positive terminal of source 30 and the negativeterminal of a different source 56. Source 56 supplies current, throughthe windings 3l and 38 of a differential relay, to the anodes 39 and 45.

The relay windings 3l, 38 are opposed to each other in their control ofa common armature 4I having contacts 42 and 43 on either side thereof.

The ultimate responsive device is not shown in the drawing, but may beconsidered as one which is operative under control of a common audiofrequency amplifier 44. This amplier derives its input energy from oneor the other of the two amplifiers I3 and I4 depending upon theoperation of the armature 4I on the differential relay 31, 38.

Amplifier I3 has an output circuit which includes the contact 42,armature 4I and an input lead to the amplifier 44. The return circuittraverses conductor 45. Similarly the amplifier I4 has an output circuitwhich includes contact 43, armature 4I, one input terminal fromamplifier 44, and a return circuit indicated by conductor 46.

With receivers A and B adjusted to have the same overall gain, and withpotentiometer 3| properly adjusted to compensate for differences in tubecharacteristics, armature 4I of the differential relay will be actuatedin accordance with any difference existing between the input voltages tothe two receivers A and B.

The functioning of the system will now be explained in more detail.Consider first the case in which receiver A (comprising the units fedsuccessively with energy from antenna I) has a greater input voltagethan the other receiver B.

Output from the diode detector 9 being greater, it will further depressthe output from receiver B. This is because the diode source I9 of thereceiver B must overcome the Voltage drop across the common loadresistor I'I before it can contribute any rectiiied output to the commonload circuit and therefore supply any rectified output through resistorsI2 and IE.

Resistor I5, under the conditions now considered, will have a voltagedrop across it which will swing the grid 23 .negative with respect tothe cathode 22. Anode current between the cathode 22 and anode 25 will,therefore, be cut oli'. At the same time resistor I6 will have little orno voltage drop across it, and, therefore, current will flow betweencathode 22 and anode 26.

With potentiometer 3| properly adjusted for the particular tubes in use,`the result of the condition described will be to permit current to flowin the upper portion of tube 3S, (that is, between cathode 5l land anode39), but current will be cut olf between cathode 51 and anode 49.Differential relay winding 31 is .then energized so as to pull up thearmature 4I against contact 4.2. This causes amplifier 44 to be suppliedwith modulation energy input from amplifier I3, and amplifier I4 is forthe time being switched off from the nal utilization circuit. As soon asconditions are so changed, however, that the output from diode detectorI0 exceeds that of diode detector 9, the switching apparatus willreverse itself, thus causing the armature 4I to move against contact 43so that the audio frequency amplifier I4 will be connected to the.amplifier 44 supplying the final utilization circuit or device.

It is within the scope of'my invention to provide switching apparatuswhich will be effective where three or more antennas are to be used fordiversity reception. In this case the principles set forth in my Patent#2,004,126, granted June 11, 1935, may be applied. In that patent it wasshown how two selectors having differential relay windings might be madeto operate in conjunction with a third selector so as to finally delivermodulation frequency energy from whichever of three receiving sets mightbe delivering the best output signal at a given instant.

It is also within the scope of my invention to provide the improvementshereinbefore set forth in connection with a volume control vcombinedsystem such as shown in my Patent #1,849,362, dated March 15, 1932.

I claim:

1. In a diversity receiving system, a plurality of antennas, a separatereceiving circuit for each antenna, means for deriving rectified signalenergy from each said circuit, an automatic gain control device commonto said receiving circuitsv and fed with components of rectified signalenergy from the several receiving circuits, means for comparing thevalues of said components, switching means under control of thedominating one of the compared components, and a utilization deviceconnectable by said switching means to that one of said receivingcircuits which delivers the maximum intensity of signal energy.

2. The device according to'claim 1 in which the first said meanscomprises discharge tubes of the diode type.

3. The device according to claim 1 in which said switching meanscomprises a differential relay.V

4. The device according to claim 1 in which a common load resistor isprovided for absorbing said components of rectified signal energy.

5. In combination with a plurality of heterodyne receivers each actuatedby a separate antenna, each receiver including a detector in circuitbetween an intermediate frequency stage and an audio frequency stage, anautomatic volume control device effective to regulate said receivers inresponse to variations in the combined output energies of saiddetectors, means including balanced impedances and discharge tubes forcomparing the amplitudes of signal energy from tWo of said detectors, adifferential relay operable under control of said comparison means, autilization circuit, andV contacts selectively closeable by said relayand constituting means for connecting said utilization circuit to thataudio frequency stage which delivers the maximum signal energy.

6. In a diversity receiving system whichl uses a plurality of heterodynereceivers having amplier stages, each receiver being controlled by aseparate antenna and each being capable of delivering signals to acommon responsive device when connected thereto by switching means, themethod of applying an automatic volume control voltage tothe amplifierstages of said receiversY and of simultaneously selecting the receiverwhich delivers the maximum of rectified output power for singlycontrolling said responsive device, Which method includes deriving saidautomatic volumeA control voltage from a combination of rectifiedcomponents of energy delivered by all said receivers, comparing saidrectified components from different ones -of said receivers, and usingthe dominating energy of said components, when compared, to switch saidcommon responsive device into operative connection with said receiverwhich delivers the maximum pOWeI.

7. The method of radio signal reception which includes separatelycollecting signal energy at.

two geographically spaced positions, distinctly amplifying andrectifying the separately col`- lected energies, comparing said energiesto determine which is the greater, combining saidv energies,automatically controlling the gain in each distinct amplifying stepunder control of said combined energies, and producing a signal responseunder the exclusive control of that amplified and rectified signalenergy which is Vderived from the one position where the collectedenergy is greater as determined by said comparison step.

8. The method of radio signal reception which includes separatelycollecting signal energy at a plurality of geographically spacedpositions, distinctly amplifying and rectifying the separately collectedenergies, comparing said energies to determine which is the greatest,combining said energies, automatically controlling the gain in eachdistinct amplifying step under control of said combined energies, andproducing a signal 4response under the exclusive control of thatampliiied and rectified signal energy which is derived from the oneposition where the collected energy is greatest as determined by saidcomparison step.

9. The method of diversity reception which includes simultaneouslymaking a plurality of separate energy collections of relativelydifferent fading characteristics, separately amplifying and rectifyingthe collected energies, comparing the magnitudes of the energies thusrectified, combining the compared components of'k said energies,controlling the gain in the aforesaid amplifying step under control ofsaid combined components, and selecting for responsive utilization asingle component of rectified energy corresponding to the separateenergy collection which exceeds the others in magnitude, said selectingvstep being in accordance with the results of said comparison step.

J OI-IN B. MOORE.

